1. Technical Field:
The present invention relates to microphones of the current source type where a resistive load is normally used to convert the signal into a voltage source for further amplification and more particularly to subscriber's telephone instruments employing two terminal electret microphones of the current source type which may be employed in long or short loop telephone environments.
2. Background Art:
Since the advent of the electronic telephone and particularly since the usage of the electret microphone has become common in such electronic telephones, various circuits have been devised to provide proper amplification for the electret microphone. Many of the circuits available have one disadvantage or another such as poor frequency response, limited range of supply voltage, tendency to acoustic feedback "singing," susceptibility to radio frequency interference and incorrect impedance or current supervision properties. Some of the circuits attempt to use a minimum of components using only a single resistor for load to reduce costs, but performance frequently suffers as a result.
Many current generation high quality but low cost telephones have been developed for the business and residential telephone subscriber. Quite frequently these units employ telephone transmitters of low output that require substantial amplification of voice signals as picked up by the telephone transmitter. Often these telephone transmitters are of the electret type with an associated built in amplifier, pre-amplifier or buffer amplifier included in the handset or hand held telephone unit in which the telephone transmitter is mounted. The associated receiver also included in the same telephone handset or telephone unit is usually a standard magnetic receiver and may or may not be driven by an associated amplifier. In such telephones the voice network may be located in the handset (hand held unit) or may be located in the associated telephone base. This network may be of the traditional coupled coil type or of the more recently employed integrated circuit variety.
Most of these telephones have not taken into account the hazard of feedback seen under certain sound reflective conditions, where output from the handset receiver may be picked up by the microphone and utilized to reinforce the output of the receiver because of the normal side tone available in the voice network. This results in the condition referred to as "singing." "Singing" is defined as an isolation that occurs usually in the frequency range of from 2,000 to 3,000 Hz and is audible without holding the telephone receiver close to the subscriber's ear. This condition typically occurs only under extreme conditions when the handset is placed downward or in a reflective corner environment forming a type of resident cavity around the microphone and the receiver.
Most previous designs have been found to have this loud and objectionable singing under certain conditions which could be transmitted over the telephone line of the other party. Most early attempts to solve the problem resulted in a reduction in the transmit gain and loss of low frequency as well as high frequency signals. Obviously such signal reduction is not desirable particularly if the transmitter is to be used on anything other than a short loop environment. One method used to stop the singing involved uses a feedback loop from the output of the telephone transmitter amplifier back to its input. This particular technique because of the degradation present in feedback arrangements results in substantial loss in transmit level.